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          EEPW首頁 > 嵌入式系統(tǒng) > 設(shè)計(jì)應(yīng)用 > Android arm linux kernel啟動(dòng)流程一

          Android arm linux kernel啟動(dòng)流程一

          作者: 時(shí)間:2016-11-09 來源:網(wǎng)絡(luò) 收藏
          雖然這里的Arm Linux kernel前面加上了Android,但實(shí)際上還是和普遍Arm linux kernel啟動(dòng)的過程一樣的,這里只是結(jié)合一下Android的Makefile,講一下bootimage生成的一個(gè)過程。這篇文檔主要描述bootimage的構(gòu)造,以及kernel真正執(zhí)行前的解壓過程。

          在了解這些之前我們首先需要了解幾個(gè)名詞,這些名詞定義在/Documentation/arm/Porting里面,這里首先提到其中的幾個(gè),其余幾個(gè)會(huì)在后面kernel的執(zhí)行過程中講述:

          本文引用地址:http://cafeforensic.com/article/201611/317680.htm

          1)ZTEXTADDR boot.img運(yùn)行時(shí)候zImage的起始地址,即kernel解壓代碼的地址。這里沒有虛擬地址的概念,因?yàn)闆]有開啟MMU,所以這個(gè)地址是物理內(nèi)存的地址。解壓代碼不一定需要載入RAM才能運(yùn)行,在FLASH或者其他可尋址的媒體上都可以運(yùn)行。

          2)ZBSSADDR 解壓代碼的BSS段的地址,這里也是物理地址。

          3)ZRELADDR 這個(gè)是kernel解壓以后存放的內(nèi)存物理地址,解壓代碼執(zhí)行完成以后會(huì)跳到這個(gè)地址執(zhí)行kernel的啟動(dòng),這個(gè)地址和后面kernel運(yùn)行時(shí)候的虛擬地址滿足:__virt_to_phys(TEXTADDR) = ZRELADDR。

          4)INITRD_PHYS Initial Ram Disk存放在內(nèi)存中的物理地址,這里就是我們的ramdisk.img。

          5)INITRD_VIRT Initial Ram Disk運(yùn)行時(shí)候虛擬地址。

          6)PARAMS_PHYS 內(nèi)核啟動(dòng)的初始化參數(shù)在內(nèi)存上的物理地址。

          下面我們首先來看看boot.img的構(gòu)造,了解其中的內(nèi)容對(duì)我們了解kernel的啟動(dòng)過程是很有幫助的。首先來看看Makefile是如何產(chǎn)生我們的boot.img的:

          out/host/linux-x86/bin/mkbootimg-msm7627_ffa --kernel out/target/product/msm7627_ffa/kernel --ramdisk out/target/product/msm7627_ffa/ramdisk.img --cmdline "mem=203M console=ttyMSM2,115200n8 androidboot.hardware=qcom" --output out/target/product/msm7627_ffa/boot.img

          根據(jù)上面的命令我們可以首先看看mkbootimg-msm7627ffa這個(gè)工具的源文件:system/core/mkbootimg.c。看完之后我們就能很清晰地看到boot.img的內(nèi)部構(gòu)造,它是由boot header /kernel /ramdisk /second stage構(gòu)成的,其中前3項(xiàng)是必須的,最后一項(xiàng)是可選的。

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          /*
          +-----------------+
          | boot header | 1 page
          +-----------------+
          | kernel | n pages
          +-----------------+
          | ramdisk | m pages
          +-----------------+
          | second stage | o pages
          +-----------------+

          n = (kernel_size + page_size - 1) / page_size
          m = (ramdisk_size + page_size - 1) / page_size
          o = (second_size + page_size - 1) / page_size

          0. all entities are page_size aligned in flash
          1. kernel and ramdisk are required (size != 0)
          2. second is optional (second_size == 0 -> no second)
          3. load each element (kernel, ramdisk, second) at
          the specified physical address (kernel_addr, etc)
          4. prepare tags at tag_addr. kernel_args[] is
          appended to the kernel commandline in the tags.
          5. r0 = 0, r1 = MACHINE_TYPE, r2 = tags_addr
          6. if second_size != 0: jump to second_addr
          else: jump to kernel_addr
          */
          /*
          +-----------------+
          | boot header | 1 page
          +-----------------+
          | kernel | n pages
          +-----------------+
          | ramdisk | m pages
          +-----------------+
          | second stage | o pages
          +-----------------+

          n = (kernel_size + page_size - 1) / page_size
          m = (ramdisk_size + page_size - 1) / page_size
          o = (second_size + page_size - 1) / page_size

          0. all entities are page_size aligned in flash
          1. kernel and ramdisk are required (size != 0)
          2. second is optional (second_size == 0 -> no second)
          3. load each element (kernel, ramdisk, second) at
          the specified physical address (kernel_addr, etc)
          4. prepare tags at tag_addr. kernel_args[] is
          appended to the kernel commandline in the tags.
          5. r0 = 0, r1 = MACHINE_TYPE, r2 = tags_addr
          6. if second_size != 0: jump to second_addr
          else: jump to kernel_addr
          */

          關(guān)于boot header這個(gè)數(shù)據(jù)結(jié)構(gòu)我們需要重點(diǎn)注意,在這里我們關(guān)注其中幾個(gè)比較重要的值,這些值定義在boot/boardconfig.h里面,不同的芯片對(duì)應(yīng)vendor下不同的boardconfig,在這里我們的值分別是(分別是kernel/ramdis/tags載入ram的物理地址):

          view plaincopy to clipboardprint?
          #define PHYSICAL_DRAM_BASE 0x00200000
          #define KERNEL_ADDR (PHYSICAL_DRAM_BASE + 0x00008000)
          #define RAMDISK_ADDR (PHYSICAL_DRAM_BASE + 0x01000000)
          #define TAGS_ADDR (PHYSICAL_DRAM_BASE + 0x00000100)
          #define NEWTAGS_ADDR (PHYSICAL_DRAM_BASE + 0x00004000)
          #define PHYSICAL_DRAM_BASE 0x00200000
          #define KERNEL_ADDR (PHYSICAL_DRAM_BASE + 0x00008000)
          #define RAMDISK_ADDR (PHYSICAL_DRAM_BASE + 0x01000000)
          #define TAGS_ADDR (PHYSICAL_DRAM_BASE + 0x00000100)
          #define NEWTAGS_ADDR (PHYSICAL_DRAM_BASE + 0x00004000)

          上面這些值分別和我們開篇時(shí)候提到的那幾個(gè)名詞相對(duì)應(yīng),比如kernel_addr就是ZTEXTADDR,RAMDISK_ADDR就是INITRD_PHYS,而TAGS_ADDR就是PARAMS_PHYS。bootloader會(huì)從boot.img的分區(qū)中將kernel和ramdisk分別讀入RAM上面定義的地址中,然后就會(huì)跳到ZTEXTADDR開始執(zhí)行。

          基本了解boot.img的內(nèi)容之后我們來分別看看里面的ramdisk.img和kernel又是如何產(chǎn)生的,以及其包含的內(nèi)容。從簡(jiǎn)單的說起,我們先看看ramdisk.img,這里首先要強(qiáng)調(diào)一下這個(gè)ramdisk.img在arm linux中的作用。它在kernel啟動(dòng)過程中充當(dāng)著第一階段的文件系統(tǒng),是一個(gè)CPIO格式打成的包。通俗上來講他就是我們將生成的root目錄,用CPIO方式進(jìn)行了打包,然后在kernel啟動(dòng)過程中會(huì)被mount作為文件系統(tǒng),當(dāng)kernel啟動(dòng)完成以后會(huì)執(zhí)行init,然后將system.img再mount進(jìn)來作為Android的文件系統(tǒng)。在這里稍微解釋下這個(gè)mount的概念,所謂mount實(shí)際上就是告訴linux虛擬文件系統(tǒng)它的根目錄在哪,就是說我這個(gè)虛擬文件系統(tǒng)需要操作的那塊區(qū)域在哪,比如說ramdisk實(shí)際上是我們?cè)趦?nèi)存中的一塊區(qū)域,把它作為文件系統(tǒng)的意思實(shí)際上就是告訴虛擬文件系統(tǒng)你的根目錄就在我這里,我的起始地址賦給你,你以后就能對(duì)我進(jìn)行操作了。實(shí)際上我們也可以使用rom上的一塊區(qū)域作為根文件系統(tǒng),但是rom相對(duì)ram慢,所以這里使用ramdisk。然后我們?cè)诎裺ystem.img mount到ramdisk的system目錄,實(shí)際上就是將system.img的地址給了虛擬文件系統(tǒng),然后虛擬文件系統(tǒng)訪問system目錄的時(shí)候會(huì)重新定位到對(duì)system.img的訪問。我們可以看看makefile是如何生成它的:

          out/host/linux-x86/bin/mkbootfs out/target/product/msm7627_ffa/root | out/host/linux-x86/bin/minigzip > out/target/product/msm7627_ffa/ramdisk.img

          下面我們來看看kernel產(chǎn)生的過程,老方法,從Makefile開始/arch/arm/boot/Makefile ~

          view plaincopy to clipboardprint?
          $(obj)/Image: vmlinux FORCE
          $(call if_changed,objcopy)
          @echo Kernel: $@ is ready
          $(obj)/compressed/vmlinux: $(obj)/Image FORCE
          $(Q)$(MAKE) $(build)=$(obj)/compressed $@
          $(obj)/zImage: $(obj)/compressed/vmlinux FORCE
          $(call if_changed,objcopy)
          @echo Kernel: $@ is ready
          $(obj)/Image: vmlinux FORCE
          $(call if_changed,objcopy)
          @echo Kernel: $@ is ready
          $(obj)/compressed/vmlinux: $(obj)/Image FORCE
          $(Q)$(MAKE) $(build)=$(obj)/compressed $@
          $(obj)/zImage: $(obj)/compressed/vmlinux FORCE
          $(call if_changed,objcopy)
          @echo Kernel: $@ is ready

          我們分解地來看各個(gè)步驟,第一個(gè)是將vmlinux經(jīng)過objcopy后生成一個(gè)未經(jīng)壓縮的raw binary(Image 4M左右),這里的vmlinux是我們編譯鏈接以后生成的vmlinx,大概60多M。這里稍微說一下這個(gè)objcopy,在啟動(dòng)的時(shí)候ELF格式是沒法執(zhí)行的,ELF格式的解析是在kernel啟動(dòng)以后有了操作系統(tǒng)之后才能進(jìn)行的。因?yàn)殡m然我們編出的img雖然被編成ELF格式,但要想啟動(dòng)起來必須將其轉(zhuǎn)化成原始的二進(jìn)制格式,我們可以多照著man objcopy和OBJCOPYFLAGS :=-O binary -R .note -R .note.gnu.build-id -R .comment -S(arch/arm/Makefile)來看看這些objcopy具體做了什么事情 ~

          得到Image以后,再將這個(gè)Image跟解壓代碼合成一個(gè)vmlinux,具體的我們可以看看arch/arm/boot/compressed/Makefile:

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          $(obj)/vmlinux: $(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.o /
          $(addprefix $(obj)/, $(OBJS)) FORCE
          $(call if_changed,ld)
          @:
          $(obj)/piggy.gz: $(obj)/../Image FORCE
          $(call if_changed,gzip)
          $(obj)/piggy.o: $(obj)/piggy.gz FORCE
          $(obj)/vmlinux: $(obj)/vmlinux.lds $(obj)/$(HEAD) $(obj)/piggy.o /
          $(addprefix $(obj)/, $(OBJS)) FORCE
          $(call if_changed,ld)
          @:
          $(obj)/piggy.gz: $(obj)/../Image FORCE
          $(call if_changed,gzip)
          $(obj)/piggy.o: $(obj)/piggy.gz FORCE

          從這里我們就可以看出來實(shí)際上這個(gè)vmlinux就是將Image壓縮以后根據(jù)vmlinux.lds與解壓代碼head.o和misc.o鏈接以后生成的一個(gè)elf,而且用readelf或者objdump可以很明顯地看到解壓代碼是PIC的,所有的虛擬地址都是相對(duì)的,沒有絕對(duì)地址。這里的vmlinx.lds可以對(duì)照著后面的head.s稍微看一下~得到壓縮以后的vmlinx以后再將這個(gè)vmlinx經(jīng)過objcopy以后就得到我們的zImage了,然后拷貝到out目錄下就是我們的kernel了~~

          在這里要強(qiáng)調(diào)幾個(gè)地址,這些地址定義在arch/arm/mach-msm/makefile.boot里面,被arch/arm/boot/Makefile調(diào)用,其中zreladdr-y就是我們的kernel被解壓以后要釋放的地址了,解壓代碼跑完以后就會(huì)跳到這個(gè)地址來執(zhí)行kernel的啟動(dòng)。不過這里還有其他兩個(gè)PHYS,跟前面定義在boardconfig.h里面的值重復(fù)了,不知道這兩個(gè)值在這里定義跟前面的值是一種什么關(guān)系???

          好啦,講到這里我們基本就知道boot.img的構(gòu)成了,下面我們就從解壓的代碼開始看看arm linux kernel啟動(dòng)的一個(gè)過程,這個(gè)解壓的source就是/arch/arm/boot/compressed/head.S。要看懂這個(gè)匯編需要了解GNU ASM以及ARM匯編指令,ARM指令就不說了,ARM RVCT里面的文檔有得下,至于GNU ASM,不需要消息了解的話主要是看一下一些偽指令的含義(http://sources.redhat.com/binutils/docs-2.12/as.info/Pseudo-Ops.html#Pseudo%20Ops)

          那么我們現(xiàn)在就開始分析這個(gè)解壓的過程:

          1)bootloader會(huì)傳遞2個(gè)參數(shù)過來,分別是r1=architecture ID, r2=atags pointer。head.S從哪部分開始執(zhí)行呢,這個(gè)我們可以看看vmlinx.lds:

          view plaincopy to clipboardprint?
          ENTRY(_start)
          SECTIONS
          {
          . = 0;
          _text = .;
          .text : {
          _start = .;
          *(.start)
          *(.text)
          *(.text.*)
          *(.fixup)
          *(.gnu.warning)
          *(.rodata)
          *(.rodata.*)
          *(.glue_7)
          *(.glue_7t)
          *(.piggydata)
          . = ALIGN(4);
          }
          ENTRY(_start)
          SECTIONS
          {
          . = 0;
          _text = .;
          .text : {
          _start = .;
          *(.start)
          *(.text)
          *(.text.*)
          *(.fixup)
          *(.gnu.warning)
          *(.rodata)
          *(.rodata.*)
          *(.glue_7)
          *(.glue_7t)
          *(.piggydata)
          . = ALIGN(4);
          }

          可以看到我們最開始的section就是.start,所以我們是從start段開始執(zhí)行的。ELF對(duì)程序的入口地址是有定義的,這可以參照*.lds的語法規(guī)則里面有描述,分別是GNU LD的-E ---> *.lds里面的ENTRY定義 ---> start Symbol ---> .text section --->0。在這里是沒有這些判斷的,因?yàn)檫€沒有操作系統(tǒng),bootloader會(huì)直接跳到這個(gè)start的地址開始執(zhí)行。

          在這里稍微帶一句,如果覺得head.S看的不太舒服的話,比如有些跳轉(zhuǎn)并不知道意思,可以直接objdump vmlinx來看,dump出來的匯編的流程就比較清晰了。

          view plaincopy to clipboardprint?
          1: mov r7, r1 @ save architecture ID
          mov r8, r2 @ save atags pointer
          #ifndef __ARM_ARCH_2__
          /*
          * Booting from Angel - need to enter SVC mode and disable
          * FIQs/IRQs (numeric definitions from angel arm.h source).
          * We only do this if we were in user mode on entry.
          */
          mrs r2, cpsr @ get current mode
          tst r2, #3 @ not user?
          bne not_angel @ 如果不是
          mov r0, #0x17 @ angel_SWIreason_EnterSVC
          swi 0x123456 @ angel_SWI_ARM
          not_angel:
          mrs r2, cpsr @ turn off interrupts to
          orr r2, r2, #0xc0 @ prevent angel from running
          msr cpsr_c, r2
          1: mov r7, r1 @ save architecture ID
          mov r8, r2 @ save atags pointer
          #ifndef __ARM_ARCH_2__
          /*
          * Booting from Angel - need to enter SVC mode and disable
          * FIQs/IRQs (numeric definitions from angel arm.h source).
          * We only do this if we were in user mode on entry.
          */
          mrs r2, cpsr @ get current mode
          tst r2, #3 @ not user?
          bne not_angel @ 如果不是
          mov r0, #0x17 @ angel_SWIreason_EnterSVC
          swi 0x123456 @ angel_SWI_ARM
          not_angel:
          mrs r2, cpsr @ turn off interrupts to
          orr r2, r2, #0xc0 @ prevent angel from running
          msr cpsr_c, r2

          上面首先保存r1和r2的值,然后進(jìn)入超級(jí)用戶模式,并關(guān)閉中斷。

          view plaincopy to clipboardprint?
          .text
          adr r0, LC0
          ldmia r0, {r1, r2, r3, r4, r5, r6, ip, sp}
          subs r0, r0, r1 @ calculate the delta offset
          @ if delta is zero, we are
          beq not_relocated @ running at the address we
          @ were linked at.
          .text
          adr r0, LC0
          ldmia r0, {r1, r2, r3, r4, r5, r6, ip, sp}
          subs r0, r0, r1 @ calculate the delta offset
          @ if delta is zero, we are
          beq not_relocated @ running at the address we
          @ were linked at.

          這里首先判斷LC0當(dāng)前的運(yùn)行地址和鏈接地址是否一樣,如果一樣就不需要重定位,如果不一樣則需要進(jìn)行重定位。這里肯定是不相等的,因?yàn)槲覀兛梢酝ㄟ^objdump看到LC0的地址是0x00000138,是一個(gè)相對(duì)地址,然后adr r0, LC0 實(shí)際上就是將LC0當(dāng)前的運(yùn)行地址,而我們直接跳到ZTEXTADDR跑的,實(shí)際上PC里面現(xiàn)在的地址肯定是0x00208000以后的一個(gè)值,adr r0, LC0編譯之后實(shí)際上為add r0, pc, #208,這個(gè)208就是LC0到.text段頭部的偏移。

          view plaincopy to clipboardprint?
          add r5, r5, r0
          add r6, r6, r0
          add ip, ip, r0
          add r5, r5, r0
          add r6, r6, r0
          add ip, ip, r0

          然后就是重定位了,即都加上一個(gè)偏移,經(jīng)過重定位以后就都是絕對(duì)地址了。

          view plaincopy to clipboardprint?
          not_relocated: mov r0, #0
          1: str r0, [r2], #4 @ clear bss
          str r0, [r2], #4
          str r0, [r2], #4
          str r0, [r2], #4
          cmp r2, r3
          blo 1b
          /*
          * The C runtime environment should now be setup
          * sufficiently. Turn the cache on, set up some
          * pointers, and start decompressing.
          */
          bl cache_on
          not_relocated: mov r0, #0
          1: str r0, [r2], #4 @ clear bss
          str r0, [r2], #4
          str r0, [r2], #4
          str r0, [r2], #4
          cmp r2, r3
          blo 1b
          /*
          * The C runtime environment should now be setup
          * sufficiently. Turn the cache on, set up some
          * pointers, and start decompressing.
          */
          bl cache_on

          重定位完成以后打開cache,具體這個(gè)打開cache的過程咱沒仔細(xì)研究過,大致過程是先從C0里面讀到processor ID,然后根據(jù)ID來進(jìn)行cache_on。

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          mov r1, sp @ malloc space above stack
          add r2, sp, #0x10000 @ 64k max
          mov r1, sp @ malloc space above stack
          add r2, sp, #0x10000 @ 64k max

          解壓的過程首先是在堆棧之上申請(qǐng)一個(gè)空間

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          /*
          * Check to see if we will overwrite ourselves.
          * r4 = final kernel address
          * r5 = start of this image
          * r2 = end of malloc space (and therefore this image)
          * We basically want:
          * r4 >= r2 -> OK
          * r4 + image length <= r5 -> OK
          */
          cmp r4, r2
          bhs wont_overwrite
          sub r3, sp, r5 @ > compressed kernel size
          add r0, r4, r3, lsl #2 @ allow for 4x expansion
          cmp r0, r5
          bls wont_overwrite
          mov r5, r2 @ decompress after malloc space
          mov r0, r5
          mov r3, r7
          bl decompress_kernel
          add r0, r0, #127 + 128 @ alignment + stack
          bic r0, r0, #127 @ align the kernel length
          /*
          * Check to see if we will overwrite ourselves.
          * r4 = final kernel address
          * r5 = start of this image
          * r2 = end of malloc space (and therefore this image)
          * We basically want:
          * r4 >= r2 -> OK
          * r4 + image length <= r5 -> OK
          */
          cmp r4, r2
          bhs wont_overwrite
          sub r3, sp, r5 @ > compressed kernel size
          add r0, r4, r3, lsl #2 @ allow for 4x expansion
          cmp r0, r5
          bls wont_overwrite
          mov r5, r2 @ decompress after malloc space
          mov r0, r5
          mov r3, r7
          bl decompress_kernel
          add r0, r0, #127 + 128 @ alignment + stack
          bic r0, r0, #127 @ align the kernel length

          這個(gè)過程是判斷我們解壓出的vmlinx會(huì)不會(huì)覆蓋原來的zImage,這里的final kernel address就是解壓后的kernel要存放的地址,而start of this image則是zImage在內(nèi)存中的地址。根據(jù)我們前面的分析,現(xiàn)在這兩個(gè)地址是重復(fù)的,即都是0x00208000。同樣r2是我們申請(qǐng)的一段內(nèi)存空間,因?yàn)樗窃趕p上申請(qǐng)的,而根據(jù)vmlinx.lds我們知道stack實(shí)際上處與vmlinx的最上面,所以r4>=r2是不可能的,這里首先計(jì)算zImage的大小,然后判斷r4+r3是不是比r5小,很明顯r4和r5的值是一樣的,所以這里先將r2的值賦給r0,經(jīng)kernel先解壓到s申請(qǐng)的內(nèi)存空間上面,具體的解壓過程就不描述了,定義在misc.c里面。(這里我所說的上面是指內(nèi)存地址的高地址,默認(rèn)載入的時(shí)候從低地址往高地址寫,所以從內(nèi)存低地址開始運(yùn)行,stack處于最后面,所以成說是最上面)

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          * r0 = decompressed kernel length
          * r1-r3 = unused
          * r4 = kernel execution address
          * r5 = decompressed kernel start
          * r6 = processor ID
          * r7 = architecture ID
          * r8 = atags pointer
          * r9-r14 = corrupted
          */
          add r1, r5, r0 @ end of decompressed kernel
          adr r2, reloc_start
          ldr r3, LC1
          add r3, r2, r3
          : ldmia r2!, {r9 - r14} @ copy relocation code
          stmia r1!, {r9 - r14}
          ldmia r2!, {r9 - r14}
          stmia r1!, {r9 - r14}
          cmp r2, r3
          blo 1b
          add sp, r1, #128 @ relocate the stack
          bl cache_clean_flush
          add pc, r5, r0 @ call relocation code
          * r0 = decompressed kernel length
          * r1-r3 = unused
          * r4 = kernel execution address
          * r5 = decompressed kernel start
          * r6 = processor ID
          * r7 = architecture ID
          * r8 = atags pointer
          * r9-r14 = corrupted
          */
          add r1, r5, r0 @ end of decompressed kernel
          adr r2, reloc_start
          ldr r3, LC1
          add r3, r2, r3
          1: ldmia r2!, {r9 - r14} @ copy relocation code
          stmia r1!, {r9 - r14}
          ldmia r2!, {r9 - r14}
          stmia r1!, {r9 - r14}
          cmp r2, r3
          blo 1b
          add sp, r1, #128 @ relocate the stack
          bl cache_clean_flush
          add pc, r5, r0 @ call relocation code

          因?yàn)闆]有將kernel解壓在要求的地址,所以必須重定向,說穿了就是要將解壓的kernel拷貝到正確的地址,因?yàn)檎_的地址與zImage的地址是重合的,而要拷貝我們又要執(zhí)行zImage的重定位代碼,所以這里首先將重定位代碼reloc_start拷貝到vmlinx上面,然后再將vmlinx拷貝到正確的地址并覆蓋掉zImage。這里首先計(jì)算出解壓后的vmlinux的高地址放在r1里面,r2存放著重定位代碼的首地址,r3存放著重定位代碼的size,這樣通過拷貝就將reloc_start移動(dòng)到vmlinx后面去了,然后跳轉(zhuǎn)到重定位代碼開始執(zhí)行。

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          /*
          * All code following this line is relocatable. It is relocated by
          * the above code to the end of the decompressed kernel image and
          * executed there. During this time, we have no stacks.
          *
          * r0 = decompressed kernel length
          * r1-r3 = unused
          * r4 = kernel execution address
          * r5 = decompressed kernel start
          * r6 = processor ID
          * r7 = architecture ID
          * r8 = atags pointer
          * r9-r14 = corrupted
          */
          .align 5
          reloc_start: add r9, r5, r0
          sub r9, r9, #128 @ do not copy the stack
          debug_reloc_start
          mov r1, r4
          1:
          .rept 4
          ldmia r5!, {r0, r2, r3, r10 - r14} @ relocate kernel
          stmia r1!, {r0, r2, r3, r10 - r14}
          .endr
          cmp r5, r9
          blo 1b
          add sp, r1, #128 @ relocate the stack
          debug_reloc_end
          call_kernel: bl cache_clean_flush
          bl cache_off
          mov r0, #0 @ must be zero
          mov r1, r7 @ restore architecture number
          mov r2, r8 @ restore atags pointer
          mov pc, r4 @ call kernel
          /*
          * All code following this line is relocatable. It is relocated by
          * the above code to the end of the decompressed kernel image and
          * executed there. During this time, we have no stacks.
          *
          * r0 = decompressed kernel length
          * r1-r3 = unused
          * r4 = kernel execution address
          * r5 = decompressed kernel start
          * r6 = processor ID
          * r7 = architecture ID
          * r8 = atags pointer
          * r9-r14 = corrupted
          */
          .align 5
          reloc_start: add r9, r5, r0
          sub r9, r9, #128 @ do not copy the stack
          debug_reloc_start
          mov r1, r4
          1:
          .rept 4
          ldmia r5!, {r0, r2, r3, r10 - r14} @ relocate kernel
          stmia r1!, {r0, r2, r3, r10 - r14}
          .endr
          cmp r5, r9
          blo 1b
          add sp, r1, #128 @ relocate the stack
          debug_reloc_end
          call_kernel: bl cache_clean_flush
          bl cache_off
          mov r0, #0 @ must be zero
          mov r1, r7 @ restore architecture number
          mov r2, r8 @ restore atags pointer
          mov pc, r4 @ call kernel

          這里就是將vmlinx拷貝到正確的地址了,拷貝到正確的位置以后,就將kernel的首地址賦給PC,然后就跳轉(zhuǎn)到真正kernel啟動(dòng)的過程~~

          最后我們來總結(jié)一下一個(gè)基本的過程:

          1)當(dāng)bootloader要從分區(qū)中數(shù)據(jù)讀到內(nèi)存中來的時(shí)候,這里涉及最重要的兩個(gè)地址,一個(gè)就是ZTEXTADDR還有一個(gè)是INITRD_PHYS。不管用什么方式來生成IMG都要讓bootloader有方法知道這些參數(shù),不然就不知道應(yīng)該將數(shù)據(jù)從FLASH讀入以后放在什么地方,下一步也不知道從哪個(gè)地方開始執(zhí)行了;

          2)bootloader將IMG載入RAM以后,并跳到zImage的地址開始解壓的時(shí)候,這里就涉及到另外一個(gè)重要的參數(shù),那就是ZRELADDR,就是解壓后的kernel應(yīng)該放在哪。這個(gè)參數(shù)一般都是arch/arm/mach-xxx下面的Makefile.boot來提供的;

          3)另外現(xiàn)在解壓的代碼head.S和misc.c一般都會(huì)以PIC的方式來編譯,這樣載入RAM在任何地方都可以運(yùn)行,這里涉及到兩次沖定位的過程,基本上這個(gè)重定位的過程在ARM上都是差不多一樣的。



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